Over the prairie garden at St. Clair Street, this August you can see up to 7 monarchs nectaring and circling one another in the air. At the much larger Odana Golf Course prairie, you can see four monarchs. What's going on?
Let's start with observations. Here's what you can easily see during a few hours:
- Males are much more common than females at these two locations. Females tend to be more widely spread out, because milkweed plants (where they lay eggs) are widely distributed.
- Males "patrol" an area with nectar plants or milkweeds (or both), flying fast around the area not far above the foliage. Occasionally they stop briefly to rest or gather nectar.
- Females fly a bit lower and more slowly, sometimes dipping into the foliage to lay eggs on milkweed, or to nectar. You can make a good guess about a monarch's sex by watching how they fly.
- If a monarch sees another, it may give chase.
- Monarchs of both sexes circle one another in rapid, chaotic, aerial "jousting." Sometimes up to 5 monarchs are involved.
- Sometimes other species are chased. I saw a male monarch briefly chase a swallowtail butterfly.
- These interactions can be brief or go on till the butterflies are out of sight.
- Occasionally, one of the circling butterflies will grab its "opponent" with its legs. Then both will tumble to the ground, where the male attempts to copulate.
- Rarely, a male will swoop down and "attack" a male or female monarch nectaring on a flower.
- When copulating, males attach their abdomen to the female's abdomen, using appendages called "claspers." If successful, they stay attached for up to 18 hours while the male gives the female a sperm packet that also contains nutrients. Males and females may mate multiple times during their life.
- Males can fly while attached to females. The females simply fold their wings and are carried away, sometimes to a safer place like a tree.
- Males often force themselves on females or even other males.
- Monarchs have excellent vision: they can easily spot other monarchs in the area, and can avoid small obstacles while flying swiftly about. But can they distinguish the sex of another monarch at a distance?
One male attacks another that was nectaring on the flowers to the left.
These behaviors remind me of a robin proclaiming its territory in your yard. At St. Clair Street, with up to 7 males in one place, it reminds me of prairie chickens, where many males stake out territories on a display ground, and females come in one by one to mate with the males on the best territories. But let's not jump to conclusions without really good evidence. And good evidence is hard to come by for tiny animals whirling about in the sky.
A Japanese biologist, Tsuyoshi Takeuchi, has reviewed evidence from the mating behavior of many butterfly species. He says there are two current hypotheses about what is really going on behind the mating behavior of butterflies:
- The males are defending territories--chasing away rival males to improve their own chances of mating. Or...
- The males are patrolling areas with nectar or milkweed that attract females, and simply trying to mate with any females they encounter there.
But, Takeuchi says there are several problems with these hypotheses:
- Butterflies don't have weapons like horns, teeth, or claws. So they can't impose a penalty on the loser of a territorial "battle." So maybe there is no "loser," no "territory," and no "battle."
- There is no evidence that--using vision--monarch males can distinguish the sex of another monarch at a distance. When evidence is lacking, it's best to assume they can't.
So Takeuchi proposes a third (and more simple) hypothesis to explain our observations of most butterflies with aerial jousting: The males are simply employing "trial and error," chasing any monarch that passes by. They circle one another until one of the males decides the chances the other is a female are poor. That male then stops circling, and the encounter ends. He calls this "the erroneous courtship hypothesis."
If, on the other hand, one of the circling monarchs is a female, circling continues until the male eventually forces her down, or she may decide to land. Copulation ensues.
If you search the internet, you'll find both hypotheses 1 and 2 above as explanations for monarch mating behavior. But given the difficulty of studying monarch mating and the lack of evidence they can distinguish sex at a distance, I'll put my money on Takeuchi's explanation... for now. Takeuchi's trial and error idea can explain all the behaviors I listed above.
Link to Takeuchi's scientific review paper.
Link to popular article about Takeuchi's ideas.
Exercise: Use the above list of behaviors as a checklist. How many of the behaviors can you observe?